Integrated circuits are typically formed on a semiconductor substrate such as a silicon wafer or other semiconducting material. In general, layers of various materials which are either semiconducting, conducting or insulating are utilized to form the integrated circuits. By way of example, the various materials are doped, ion implanted, deposited, etched, grown, etc. using various processes. A continuing goal in semiconductor processing is to reduce the size of individual electronic components thereby enabling smaller and denser integrated circuitry.
One type of electronic device is a field effect transistor. Such includes a pair of source/drain regions having a channel region received therebetween. A conductive gate of the transistor is received transverse the channel region, and separated therefrom by a gate dielectric region. The flow of current through the channel between the source/drain regions is controlled by the application of a potential to the transistor gate. One common type of transistor gate construction includes a combination of conductively doped polysilicon and one or more higher electrically conductive layers.
One manner of forming a transistor gate is to form a succession of layers over what will be the source/drain and channel regions. One exemplary sequence of layers is a gate dielectric layer, thereover polysilicon, thereover tungsten nitride, thereover elemental tungsten and thereover silicon nitride. The silicon nitride can function as an insulative cap for the finished gate. In certain applications, it is desirable that the polysilicon of some of the transistor gates be doped with p-type material and the polysilicon of other of the transistor gates be doped with n-type material. Typically in such instances, the polysilicon layer is initially deposited in an undoped form. Thereafter, different portions of the polysilicon are masked and ion implanted with their respective p-type or n-type conductivity enhancing impurity atoms. The substrate is then annealed, for example by rapid thermal processing (RTP), effective to electrically activate the implanted impurity atoms received within the polysilicon layer.
Typically, photoresist is then deposited and patterned over the silicon nitride layer into a desired pattern. In one existing method, the photoresist is used as a mask while etching the desired pattern through the silicon nitride layer, the tungsten layer, and the tungsten nitride layer, but only partially into the activated ion implanted polysilicon. The typical anisotropic etching utilizes plasma and can use different chemistries depending upon the particular material being etched. The desired etching chemistry and conditions for etching completely through the polysilicon can have a tendency to damage or attack the gate sidewalls being formed and created by the etch of the tungsten and tungsten nitride layers. Accordingly, the photoresist is removed and a thin silicon nitride layer is typically deposited and anisotropically etched to form thin insulative sidewall spacers over the tungsten and tungsten nitride sidewalls as well as over the partially etched polysilicon. The etch can then be completed through the polysilicon using the silicon nitride cap as an etch mask, with the sidewall spacers protecting the tungsten and tungsten nitride sidewalls during the remaining polysilicon etch.
Unfortunately, the etch through the tungsten layers which goes partially into the polysilicon has been discovered to create undesirable wide and deep cracks in the outer polysilicon surface. The subsequently deposited silicon nitride tends to fill these cracks. The anisotropic etch to form the thin silicon nitride sidewall spacers may not remove all of the silicon nitride from within the cracks of the exposed polysilicon layer. Accordingly, the subsequent etch of the polysilicon has to also contend with the silicon nitride remaining in the cracks, and can lead to less-than-complete etching of the polysilicon, as well as other problems.
While the invention was motivated in addressing the above issues, it is in no way so limited. The invention is only limited by the accompanying claims as literally worded (without interpretative or other limiting reference to the above background art description, remaining portions of the specification or the drawings) and in accordance with the doctrine of equivalents.